1. Field of the Invention
This invention relates to an optical device, and particularly but not exclusively to a device for modulating radiation guided in a waveguide.
2. Discussion of Prior Art
Optical devices are well known in the prior art. They are described in a publication xe2x80x9cIntroduction to Semiconductor Integrated Opticsxe2x80x9d by H P Zappe (ISBN 0-89006-789-9, Artech House Publishers 1995). Optical devices for modulating radiation operate by exploiting optical properties of a modulating medium which are modifiable by external influences. One of the optical properties may include a refractive index. Induced changes in the refractive index may be anisotropic, where the medium becomes birefringent, or isotropic. There are many possible techniques for modulating the refractive index. These techniques are herewith described.
Refractive index changes may be induced in some optically transmissive materials by the application of an external mechanical force to them. This is referred to as a photo-elastic effect. Thermally induced refractive index changes are referred to as a thermo-optic effect.
Magnetically induced birefringence, referred to as a Faraday or magneto-optic effect, arises within some optically transmissive materials when subjected to a magnetic field. Factors such as magnetic flux density within the materials, a Verdet constant of the materials, composition of the materials and radiation propagation path length within the materials determine the magnitude of birefringence attainable.
Refractive index changes may be induced in some materials by application of an electric field to them. These refractive index changes occur due to both the Kerr and the Pockels effect. Refractive index changes arising from the Kerr effect are proportional to the Kerr constant of the materials and the square of the electric field applied to them. For the Pockels effect, refractive index changes are proportional to the applied electric field. The Pockels effect is only observed in crystalline materials comprising crystals which lack a centre of symmetry.
Refractive index changes may also be induced in some materials by introducing free charge carriers into them. Such changes are referred to as free carrier modulation or sometimes as a plasma dispersion effect. The free carriers modify both real and imaginary parts of the refractive index, thereby introducing both optical phase shift and optical absorption to optical radiation propagating through regions of these materials in which the carriers are present.
Silicon has a centro-symmetric crystalline structure and therefore does not exhibit the Pockels effect, except when high temperature poling is applied in which case a weak effect is obtained. This weak effect corresponds to a coefficient r of 10xe2x88x9212 m Vxe2x88x921 in equation [1] which describes a change in refractive index xcex94n as a function of silicon refractive index no and applied electric field E:                               Δ          ⁢                      xe2x80x83                    ⁢          n                =                              1            2                    ⁢                      n            0            3                    ⁢          rE                                    [        1        ]            
Silicon weakly exhibits the Kerr effect when very high strength electric fields are applied to it, for example refractive index changes of approximately 10xe2x88x924 are attainable for applied electric field strengths of 106 V mxe2x88x921. In order to provide a practicable optical device for modulating radiation based upon a silicon waveguide, either the thermo-optic effect or the plasma dispersion effect have to be exploited. Operating bandwidths of devices relying on the thermo-optic effect in a silicon waveguide are restricted by relatively slow thermal dynamics of the waveguide, bandwidths of tens of kilohertz may be attained in practice for power inputs amounting to several Watts. Conversely, operating bandwidths of devices relying on the plasma dispersion effect in silicon waveguide are restricted by rapidity of removal and injection of charge carriers from a region thereof in which optical radiation propagates; such devices may provide operating bandwidths of several tens of megahertz in practice.
Optical radiation propagating within a homogeneous medium has an electric field vector of a magnitude E which varies spatially in the medium at an instance of time according to equation [2]:
xe2x80x83Exe2x88x9deikxxe2x80x83xe2x80x83[2]
in which
k is a wavenumber of the optical radiation;
x is a distance in the medium; and
i is a square root of xe2x88x921.
The wave number k in equation [2] is expressible as a product of a free-space wavenumber ko for the optical radiation and the refractive index n of the medium according to equation [3]:
Exe2x88x9deinkoxxe2x80x83xe2x80x83[3]
In equation [3], the refractive index n may be expressed in terms of a real part nr and an imaginary part xcex1 according to equation [4]:
n=nr+ixcex1xe2x80x83xe2x80x83[4]
from which the magnitude of the electric field strength E is expressible according to equation [5]:
Exe2x88x9deinrkoxexe2x88x92xcex1koxxe2x80x83xe2x80x83[5]
When the medium is silicon, injection of free carriers thereinto modifies both the real part nr and imaginary part xcex1 of the refractive index n which are interrelated according to the Kramers-Kronig relationship which is expressed in equations [6] and [7]:                               Δ          ⁢                      xe2x80x83                    ⁢                      n            r                          =                              -                                                            q                  3                                ⁢                                  λ                  2                                                            4                ⁢                                  π                  2                                ⁢                                  c                  3                                ⁢                                  n                  r                                ⁢                                  ϵ                  o                                                              ⁢                      (                                                            N                  e                                                                      m                    ce                    2                                    ⁢                                      μ                    e                                                              +                                                N                  h                                                                      m                    ch                    2                                    ⁢                                      μ                    h                                                                        )                                              [        6        ]                                          Δ          ⁢                      xe2x80x83                    ⁢          α                =                              -                                                            q                  2                                ⁢                                  λ                  2                                                            8                ⁢                                  π                  2                                ⁢                                  c                  2                                ⁢                                  n                  r                  2                                ⁢                                  xe2x80x83                                ⁢                                  ϵ                  o                                                              ⁢                      (                                                            N                  e                                                  m                  ce                                            +                                                N                  h                                                  m                  ch                                                      )                                              [        7        ]            
in which
c is the speed of light in vacuum;
xcexce is an electron mobility within silicon;
xcexch is an hole mobility within silicon;
mce is an effective mass of a free electron within silicon;
mch is an effective mass of a free hole within silicon;
q is the charge on an electron;
xcex is a wavelength of radiation propagating in the medium;
Ne is a free electron concentration within the medium;
Nh is a free hole concentration within the medium;
xcex94nr is a change in the real part nr;
xcex94xcex1 is a change in the imaginary part xcex1; and
xcex5o is the permittivity of free space.
For optical radiation of 1 xcexcm wavelength propagating in silicon, changes to the real part nr of approximately 10xe2x88x924 may be induced by charge carrier injection. Accompanying changes to the imaginary part are an order of magnitude smaller than this.
Prior art optical devices for modulating radiation based on a silicon waveguide generally exploit the plasma dispersion effect. Such devices employ a silicon p-i-n diode structure fabricated using standard silicon microfabrication techniques, for example epitaxial techniques for growing layers onto a wafer substrate. The structure comprises an electron acceptor doped p region, an intrinsic i region in the form of a rib and an electron donor doped n region. Optical radiation is confined to the intrinsic i region which functions as a waveguide. Charge carriers are injected into the intrinsic i region from the p and n regions when the p region is biased at a higher potential than the n region. The carriers modulate the refractive index of the waveguide.
The injected charge carriers induce a small phase change in the radiation propagating in the prior art devices. This phase change is converted into an amplitude change by incorporating at least one device into a Mach-Zehnder interferometer.
A first example of a prior art optical device is described in a patent specification U.S. Pat. No. 4,787,691. The device is designed for modulating and switching guided light in a waveguide. It comprises in sequence a silicon substrate base, a n+ doped influx silicon substrate, a low refractive index dielectric layer, a n-type crystalline silicon layer and a p+ doped silicon layer. The low index dielectric layer is etched during device fabrication to form a dielectric strip in the device. The n-type layer and p+ doped layer are etched during device fabrication to form a waveguide with a p+ electrode on top of it, said waveguide and electrode aligned along the strip. The strip assists to confine radiation within the waveguide. The p+ electrode forms a first electrode of the device and the substrate base forms a second electrode thereof. A potential difference applied between the first and second electrodes results in carrier injection into the waveguide which modifies its refractive index and hence characteristics of radiation propagating therealong.
The device described above in the specification U.S. Pat. No. 4,787,691 is fabricated using a process which involves etching layers grown onto the substrate base. Its structure is therefore governed by limitations imposed by the process. One of these limitations is that the substrate base is used for one of the electrodes. This results in a first problem when several devices are formed together on the substrate base that the base will form a second electrode common to the devices. This places limitations on circuit configuration possible for controlling the device. Moreover, the base and the influx silicon substrate provide a conductivity which is several orders of magnitude less than that of a metal such as aluminium. This results in a second problem of electrode series resistance which degrades device operating efficiency because power is dissipated within the series resistance itself rather than in regions where charge injection occurs and a useful modulation effect is obtained. Furthermore, charge carriers in the device are injected predominantly into edge regions of the waveguide on account of the dielectric strip being positioned beneath the waveguide. However, radiation propagates predominantly in a central region of the waveguide hence charge carriers injected into edge regions of the waveguide are not particularly effective at modulating radiation in the waveguide. Therefore, an unnecessary excess of charge carriers are injected to achieve a desired modulation of radiation within the waveguide. This results in a third problem that the unnecessary excess of carriers reduces modulation bandwidth of the device because of time required for recombination of the excess of carriers within the waveguide.
A second example of a prior art optical device is described in a European patent specification EP 0 121 401 A2. The device comprises in sequence a substrate, a substrate layer, an optical waveguide layer and buffer layers formed of either all n-type or all p-type compound semiconductor crystal. The layers are all formed by epitaxial deposition onto a first side of the substrate. A rib waveguide is formed from the buffer layers by selectively etching them. One of the buffer layers provides a first electrode on top of the rib waveguide and a metal alloy layer deposited on a second side of the substrate provides a second electrode. Radiation propagating along the waveguide is modified in response to a potential difference applied to the first and second electrodes. The device described above in the European patent specification suffers, on account of limitations arising from its method of fabrication, from the first and second problems mentioned above affecting the device in the first example.
A third example of a prior art optical device is described in a patent specification U.S. Pat. No. 4,093,345. The device incorporates a monocrystalline substrate of n-type gallium arsenide supporting a first epitaxial layer of n-type aluminium gallium arsenide, a second epitaxial layer of n-type aluminium gallium arsenide having a lower aluminium-to-gallium ratio than that of the first epitaxial layer, a layer of electrode cladding material contacting a rib portion of the second epitaxial layer, a gold electrode contact layer ohmically contacting the electrode cladding layer and a tin electrode contact layer ohmically contacting the substrate. The device is arranged so that a modulating potential applied to the gold electrode layer and the tin contact layer changes refractive index of the rib portion for modulating radiation propagating therealong. The device is fabricated using an epitaxial process which imposes limitations on structure of the device. As a result of these limitations, the device suffers from the first and second problems mentioned above which affect the devices described in the first and second examples above.
It is an object of the invention to provide an alternative optical device which alleviates at least one of the problems mentioned above.
According to the present invention, an optical device is provided which has an active region for radiation propagation and injecting means for injecting charge carriers into the active region, characterised in that the injecting means incorporates a high conductivity buried layer between two wafer elements of a bonded wafer couplet and the device incorporates concentrating means between the buried layer and the active region for concentration of charge carriers in the active region.
The invention provides an advantage that the high conductivity layer provides an electrical path for biasing the device with reduced dissipation compared to prior art optical devices. Moreover, the invention provides an advantage that the device modulates radiation more effectively than prior art devices because the concentrating means concentrates charge carriers in the active region where radiation propagates.
The device may incorporate a dielectric insulating layer for electrically isolating it within the wafer couplet. This provides an advantage, for example when several devices are fabricated together on the couplet, that the device is isolated from the wafer elements.
The active region may incorporate dopant impurity to a concentration to a concentration of less than 1016 atoms cmxe2x88x923. This provides an advantage that the active region is capable of providing a propagation path for radiation where radiation attenuation is less than 1 dB cmxe2x88x921.
The active region may provide radiation waveguiding means with refractive index modulatable by the injecting means. This provides a convenient device configuration for modulating radiation propagating in the active region, especially when the active region comprises material having a centro-symmetrical crystal structure.
The concentrating means may comprise a first electrode located upon one side of the active region and the device includes a second electrode located upon the other side. This provides an advantage of being a simple practical configuration for the device.
In a first embodiment, the concentrating means may be a region of the buried layer which projects through an insulating layer extending between parts of the device. This provides a structure which is particularly effective at concentrating charge carriers in the active region, thereby increasing effectiveness of the device.
The buried layer may be a polysilicon layer. This provides an advantage that polysilicon is a convenient material to use for the layer because it is easy to deposit using conventional semiconductor fabrication equipment.
The polysilicon layer may incorporate dopant impurity to a concentration in a range of 1018 to 1019 atoms cmxe2x88x923. Employing a dopant concentration in this range is advantageous because it is achievable using conventional semiconductor fabrication processes.
In a second embodiment, the concentrating means may be a heavily doped region of different chemical composition to the buried layer. This provides an advantage that the concentrating means may be preferentially adapted for injecting charge carriers into the active region and the buried layer may be preferentially adapted for providing an electrical connection path to the concentrating means.
The buried layer may be a metal silicide layer. This provides an advantage that the silicide layer has a coefficient of resistivity of less than 1.5 xcexcxcexa9m and thereby provides a reduced resistance connection path to the concentrating means compared to the prior art, thereby resulting in reduced device operating dissipation.
The buried layer may be a tungsten suicide layer. This provides an advantage that tungsten silicide is capable of withstanding high temperatures in the order of 1000xc2x0 C. required for performing subsequent processing steps for fabricating the device.
The buried layer may be any one of tantalum silicide layer, a cobalt silicide layer and a titanium silicide layer. This provides an advantage of a range of materials which may be preferentially employed for fabricating the buried layer.
The concentrating means and the buried layer may share a like dopant impurity providing conductivity in the former. This provides an advantage that the concentrating means may be selectively doped and act as a source of dopant during device fabrication.
In another aspect of the invention, a method of fabricating a device of the invention may include the steps of:
(a) providing first and second wafer elements;
(b) providing the wafer elements with a layer structured to define injecting means for injecting charge carriers into an active device region for radiation propagation;
(c) providing one of the wafer elements with a metal silicide or a polysilicon layer to provide injecting means;
(d) bonding the wafer elements to form a wafer couplet within which the metal silicide layer or the polysilicon layer is buried; and
(e) processing the couplet to define the active device region.
The method provides an advantage of providing a process for fabricating the device which is not possible to fabricate using conventional prior art techniques, for example fabrication of the device is not presently feasible using epitaxial techniques to deposit successive layers onto a wafer.
In another aspect of the invention, a device according to the invention may be fabricated by using the method referred to above.